Heat engines are used to convert heat or thermal energy into useful mechanical work and are often used in power generation plants. One example of a heat engine is an expander-generator system which generally includes an expander (e.g., a turbine) rotatably coupled to a generator or other power converting device. As working fluids are expanded in the expander, the shaft connecting the turbine and generator rotates and generates electricity in the generator.
Most power plant expander-generators are based on the Rankine cycle and obtain high temperature/pressure working fluids through the combustion of coal, natural gas, oil, and/or nuclear fission. Typical working fluids for Rankine cycles include water (steam) and organic fluids. Recently, however, due to perceived benefits in terms of hardware compactness, efficiency, heat transfer characteristics, etc., there has been considerable interest in using super-critical carbon dioxide (ScCO2) as a working fluid for certain heat engine applications. Notable among such applications are nuclear, solar, and waste heat energy conversion cycles.
Many waste heat recovery schemes utilizing ScCO2 as the working fluid have relatively high process temperatures/pressures. For example, in some applications the temperatures can approach or exceed 600° C. and the operating pressures can range from between 1000 and 3000 psia. Elevated operating pressures can potentially impose large axial thrusts on equipment rotors, which may damage accompanying bearing systems if not properly managed. Axial thrusts are conventionally controlled using several methods, including: using a balance piston at the high pressure section, varying the rotor diameter in each section, and/or varying the number of expansion stages that make up each section. These methods, however, sacrifice valuable rotor axial length and otherwise increase the overall weight of the machine, thereby negatively affecting the efficiency of the machine.
What is needed, therefore, is a compact expander-generator that effectively cools the rotating equipment and simultaneously counteracts axial thrusts without sacrificing rotor length or unnecessarily adding to the overall weight of the machine.